In the alternate timeline that was greatly affected by the Cold War, they became enemies of the Certoss, with the Na'kuhl tampering Earth's history and making Humans a threat to the Certoss, which led to the destruction of Certoss Ajahlan in that timeline's future. This led to some Certoss from that future to travel through time to stop the Na'kuhl and wipe out humanity, even when the war was effectively over in the primary universe and the timeline was restored. (TOSnovel: From History's Shadow)

At some point, a Na'kuhl faction contacted the Klingons, and gave some of them knowledge of the future in an attempt to manipulate them into doing their bidding. However, the Klingons rejected this and rebelled against the Na'kuhl. Among the knowledge they had gained was the location of a Doomsday machine, which B'Vat later found in the T'Ong Nebula. (STOmission: "The Doomsday Device")